It has been well established that operation of an internal combustion engine in homogeneous, lean mode results in a large decrease of NOx and particulate matter emissions. Fundamentally, the reason for the decrease in these emissions is that homogeneous, lean mode engine operation avoids either fuel rich regions that generate particulate matter emissions or stoichiometric conditions that generate high temperature and NOx emissions. One type of engine operation in a homogeneous, lean mode is homogeneous charge compression ignition (HCCI) which involves the compression of a fuel/air mixture until the mixture self ignites. (See, for example, PCT Publication WO 99/42718, the teachings of which are incorporated herein by reference.)
The control of HCCI engine operation has been explored. Successful operation has been obtained, for some operating regions, by the use of late injection into an engine cylinder, with long ignition times (longer than the times required for fuel injection and for the establishment of a relatively homogeneous charge). Some degree of control through stratification of the temperature or fuel charge has been achieved, either by the use of recycled exhaust gas (EGR), or by using a heat exchanger to preheat the incoming air using hot exhaust gases. The difficulty with this approach is that ignition timing is a strong function of the temperature of the incoming air, as sensitive as 1 crank-angle degree ignition timing per 3–4° C. difference in the air charge temperature. Precise control of the temperature is therefore required for optimum results. In addition, for transportation applications with the presence of transients, a very fast response is required, especially at conditions of high power characteristic of merging and passing. Other methods of control in HCCI involve variable valve timing (VVT) or variable compression ratio (VCR), but both methods require sophisticated engine actuators.
The self-ignition timing in HCCI operation is a strong function of the fuel octane rating. Experiments with HCCI operation using fuels with several values of RON (Research Octane Number) have discovered that fuels with lower RON (i.e., more prone to self ignition) have a rather broad operational regime, as opposed to those with a high octane value. An alternative theory suggests that it is the cetane number, not the octane number, that determines the self-ignition timing.
Additives to either promote oxidation (ozone) or to ignite the fuel (by variation of the octane rating or cetane numbers of the fuel) have also been proposed. The onboard generation of ozone requires electrical power, with control being very sensitive to the amount of ozone. A 10 ppm concentration of ozone results in a 1 degree change in the ignition timing. The variation of the fuel octane/cetane numbers requires two or more fuels on board, with the inconvenience of carrying and refueling two or more tanks. Mixtures of methane and either DME, nafta, FisherTropsch fuels, and others have been explored for octane/cetane number variation. This method of ignition control in HCCI is referred to as fuel blending. Distinct fuels, from separate containers, have been used and fuel reforming has been proposed for the onboard generation of the secondary fuel required in the fuel blending process. Ignition improvers can also be used to change the cetane number of the fuel, and therefore alter the ignition characteristics of the fuel, and could be used for ignition control of HCCI.
Options for fuel reforming technology have been considered. A reformer chemically transforms fuel from one form to another. As practiced in the field today, fuel reformers are generally of a catalytic nature. A preferred method for fuel reforming on board vehicles is partial oxidation where a given amount of air is mixed with a given amount of fuel and in which the oxygen content is substantially lower than that required for full combustion of the fuel.
Under ideal stoichiometric partial oxidation conditions, the partial oxidation reaction for reforming hydrocarbon fuel (with air) is:CnHm+n/2O2+2nN2→nCO+m/2H2+2nN2In this case there is just enough oxygen around to convert all the carbon in the fuel into CO. The partial oxidation reaction is exothermic. In the case of liquids fuels (gasoline, diesel), approximately 15% of the heating value of the fuel is released in the partial oxidation reaction. Operation requires that sufficient oxygen be present to prevent the formation of soot particles, but with low amounts of oxygen to maximize the heating value of the hydrogen rich gas. It is possible to use exhaust gas as the oxidant. In order to do so, it is preferable that the engine operates in a lean mode, so that there is some free oxygen in the engine exhaust.
Reformers can be used to generate fuels of different octane value. When operating at oxygen/carbon ratio (O/C)>1.2, the reformate has a composition similar to synthesis gas, with about 20 mol % H2 and CO, and small concentration of CO2, C2's, water, and the balance nitrogen. The synthesis gas can be used to increase the octane value of the fuel (since hydrogen has a very high octane value, while CO is similar to that of methane), but requires its use not as an additive but as a substantial fraction of the fuel.
When the O/C<1.2, it is possible to generate substantial concentration of C2 compounds, which have low octane number. In this case, the reformate composition may include a ethylene concentration that can be as high as a few percent. The concentration of hydrogen and CO is on the order of 20 mol %, but since the heating value per mol of ethylene is about 4 times that of hydrogen or CO, the contribution to the energy content in the reformate due to C2's is substantial. Under ideal circumstances, the process can be written as:CnH2n+m/2O2+2mN2→mCO+mH2+(n−m)/2C2H4+2mN2In practice, the process is somewhat less efficient, but for the process of illustration the above is adequate.
It has been suggested to use EGR to the control of the rate of heat release in the engine, needed to avoid knock, by the establishment of substantial temperature gradients in the cylinder. (See, for example, A numerical study to control combustion duration of hydrogen fueled HCCI by using multi-zone chemical kinetics simulations, Noda T and Foster, D., SAE 2001-01-0250, the teachings of which are incorporated herein by reference.) However, it has not been previously recognized that reformate gas may be used to control the rate of heat release through established temperature non-uniformities. Nor has there been any mention of using the thermal content of the reformate, as the partial oxidation reforming process is an exothermic reaction, for thermal management of the air/fuel charge into the cylinder.
There is therefore a need for a engine operating in a HCCI mode where reformate gas produced by an onboard fuel reformer from fuel, air, exhaust and other reagents is stored onboard and used for control of engine operation.